1. Field of the Invention
This invention relates to signal detector circuits and, more particularly, to sensitive detectors for telephone loop signaling.
2. Description of the Prior Art
It is common practice to utilize sensitive relays to detect signaling currents in telephone subscriber loops. Thus, when a subscriber goes off-hook, the resulting current flow in the subscriber loop causes a line relay to operate and signal the central office of the off-hook condition. Similarly, loop interruptions caused by a rotary dial are detected in a dial pulse receiver, also including a sensitive relay responsive to interruptions in loop currents.
On the other hand, for subscribers who are being called rather than initiating a call, ringing signals are applied to the subscriber's loop to operate the telephone ringer. When the called subscriber goes off-hook, the resulting loop current flow is detected by a ring-trip relay to interrupt the ringing signals on the loop. This function, called "ring-trip," requires the detection of a low level dc loop current in the presence of a high level ac ringing signal.
Electronic loop current detectors, such as that shown in the copending application of S. J. Brolin and S. Colodner, now U.S. Pat. No. 4,056,690, granted Nov. 1, 1976, and assigned to applicant's assignee, are known. Adequate discrimination against dial pulse splitting and adequate threshold stability remain problems, however.
In many subscriber loop carrier systems, such as those shown in J. L. Caldwell U.S. Pat. No. 3,963,869, granted June 15, 1976, and the copending application of T. N. Rao-R. Toumani, now U.S. Pat. No. 4,028,628, granted June 7, 1977, telephone supervisory and loop closure signals cannot be transmitted directly through the carrier system. These signals must therefore be detected at the remote end of the carrier system and transmitted through the system to the central office terminal.
The usefulness of such carrier systems is dependent, in part, on how far the subscriber loops can be extended from the remote end of the system. In order to extend such loops beyond the remote terminal for substantial distances, a sensitive and discriminating detector for off-hook, dial pulse and ring-trip currents is required. Moreover, the detector must provide accurate detection of these signals without significantly distorting the dial pulses.